The present invention relates generally to the field of combustion and flue gas cleanup methods and apparatus and, in particular, to a new and useful method and apparatus for removing mercury from the flue gases generated during the combustion of fossil fuels such as coal, or solid wastes, through the use of hydrogen sulfide.
In recent years, the U.S. Department of Energy (DOE) and the U.S. Environmental Protection Agency (EPA) have supported research to measure and control the emissions of Hazardous Air Pollutants (HAPs) from coal-fired utility boilers and waste to energy plants. The initial results of several research projects showed that the emissions of heavy metals and volatile organic carbons (VOCs) are very low, except for mercury (Hg). Unlike most of the other metals, most of the mercury remains in the vapor phase and does not condense onto fly ash particles at temperatures typically used in electrostatic precipitators and fabric filters. Therefore, it cannot be collected and disposed of along with fly ash like the other metals. To complicate matters, mercury can exist in its oxidized (Hg+2) form, principally as mercuric chloride, (HgCl2), or in its elemental (Hg0) form as vaporous metallic mercury. The relative amount of each species appears to depend on several factors such as fuel type, boiler combustion efficiency, the type of particulate collector installed, and various other factors.
The search for industrially acceptable methods for the capture of mercury from industrial flue gases has included a significant effort to determine how much mercury can be removed by existing, conventional air pollution control equipment, such as wet or dry scrubbers.
Accordingly, tests have been performed on several commercial scale and pilot scale wet scrubbers. In addition to being applicable to dry scrubber situations, these tests have produced some expected and some surprising results. It was generally expected that the oxidized mercury would be easily captured and the elemental mercury would be difficult to capture. These expectations were based on the high solubility of mercuric chloride in water and the very low solubility of elemental mercury in water. This expectation was generally fulfilled.
The surprising result concerned elemental mercury. Repeated tests during which the concentration of elemental mercury in the flue gas was measured revealed that more elemental mercury was leaving the wet scrubber than was entering.
One postulate proposed to explain the cause of the elemental mercury generation in the wet scrubber is described for example, by the following general reactions:
Mex+Hg+2xe2x86x92Mex+2+Hg0
2Mex+Hg+2xe2x86x922Mex+1+Hg0
Me is any number of transition metals such as Fe, Mn, Co, etc., that may be present in one of several possible oxidation states, x.
Transition metal ions are generally present in wet scrubber slurries as impurities in the industrial applications of concern. Thus, as the mercuric chloride is absorbed, a portion reacts with and becomes reduced by trace levels of transition metals and metal ions and because of its low solubility the elemental mercury is stripped from the liquid and returned to the flue gas.
Most of the recent efforts to capture and remove mercury from the flue gas produced by coal-fired units have concentrated on gas-phase reactions with introduced reagents such as activated carbon.
The subject of mercury emissions by the utility and waste to energy industries is a new area being investigated by both the DOE and EPA.
The present invention provides a means in a wet or dry scrubber to rapidly precipitate the oxidized mercury at the gas/liquid interface in the scrubber before it can be reduced by the transition metals. One of the most insoluble forms of mercury is mercuric sulfide, which in mineral form is cinnabar. One means for supplying a source of sulfide to react with the oxidized mercury is aqueous sulfide ions. Thus, at the gas/liquid interface in the scrubber, the following reaction is proposed for the absorption and precipitation of ionized (oxidized) mercury:
Sxe2x88x922(aq)+HgCl2(g)xe2x86x92HgS(s)+2 Clxe2x88x92(aq)
HgS has a solubility product of 3xc3x9710xe2x88x9252 and therefore precipitates essentially completely. The aqueous sulfide species is added to the scrubbing liquor of the scrubber and comes into contact with the mercury in the flue gas, such that HgS is formed when the mercury is absorbed into the liquor. Therefore, the oxidized mercury will rapidly precipitate as cinnabar in the scrubber and thereby prevent the reduction of that mercury back to vaporous elemental mercury. The precipitation of mercury as cinnabar has a distinct advantage over other mercury sequestering methods in that it converts mercury to a very insoluble form. In this way, the mercury should be inert and effectively removed from the food chain.
Accordingly, one aspect of the present invention is drawn to an improvement in a method using a scrubber for receiving and scrubbing an industrial gas containing mercury, the improvement comprising: adding an aqueous sulfide salt to the industrial gas and scrubbing the industrial gas in the scrubber. The method according to the present invention is particularly suited to the task of reducing mercury emissions in an industrial process which burns coal in a furnace to produce an exhaust flue gas, including conveying the exhaust flue gas through a dust collector, such as a fabric filter or electrostatic precipitator.
Another aspect of the present invention is drawn to an apparatus using a scrubber for receiving and scrubbing an industrial gas containing mercury with an aqueous alkali reagent, and particularly the improvement comprising: means for providing sulfide ions and means for controlling the sulfide ions provided to the industrial gas in the scrubber. The present invention is again particularly suited to utility installations which burn fossil fuels such as coal, or solid wastes, and which use a dust collector (such as an electrostatic precipitator or a fabric filter), in addition to the scrubber, and/or other conventional components for reducing emissions to the atmosphere.
Another aspect of the present invention is drawn to an apparatus for receiving and scrubbing an industrial gas containing mercury with an aqueous alkali reagent, comprising a scrubber, having a scrubber liquor, for scrubbing the industrial gas with the aqueous alkali reagent; flue means for conveying the industrial gas to the scrubber; means for providing sulfide ions; and means for controlling the sulfide ions provided to the industrial gas. The present invention is again particularly suited to utility installations which burn fossil fuels, such as coal, and may be incorporated into a wet or dry scrubber.
All aspects of the present invention contemplate means for providing sulfide ions, including but not limited to hydrosulfide (HSxe2x88x92) ions. Notably, such hydrosulfide ions (HSxe2x88x92) provide sulfide ions (Sxe2x88x922) by virtue of the equilibrium in aqueous solution:
Sxe2x88x922(aq)+H20 ⇄HSxe2x88x92(aq)+OHxe2x88x92(aq)
This means can be accomplished through the addition of an aqueous sulfide species, such as sulfidic waste water, kraft caustic liquor, kraft carbonate liquor, potassium sulfide, sodium sulfide, and/or thioacetamide, to the scrubbing liquor in the scrubber. Further, control means, such as a separate storage tank and metering pump, may be employed to selectively control the provision of sulfide to meet specific operational requirements.
This system has an inherent safety advantage in that no H2S, which is odorous and toxic, is accumulated or stored. Further, the system is versatile in that it is equally applicable to wet or dry scrubbers and may be incorporated into current emissions control systems with minimal modifications or additions.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific benefits attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated.